1. Field of the Invention
The present invention relates to an apparatus and a process for producing a zinc oxide film that forms a zinc oxide thin film on a long size substrate (simply referred to as xe2x80x9clong substratexe2x80x9d) such as a stainless steel sheet by electrodeposition, and more particularly to an apparatus and a process for producing a zinc oxide film that can effectively prevent soil (or contamination) from being generated in a bath or a rinsing tank or on a long substrate for a period of time from startup of the apparatus after one electrodeposition to subsequent electrodeposition.
2. Related Background Art
Photovoltaic elements comprised of amorphous silicon hydride, amorphous silicon germanium hydride, amorphous silicon hydride carbide, microcrystalline silicon or polycrystalline silicon are conventionally provided with reflecting layers on their backs in order to improve light-collection efficiency in the long-wavelength regions. It is desirable for such reflecting layers to show effective reflection characteristics at wavelengths which are close to band edges of semiconductor materials and at which absorption becomes small, i.e., wavelengths of 800 nm to 1,200. Those which can fulfill such a condition are metals such as gold, silver, copper, aluminum, etc.
It is also prevalent to provide an uneven layer which is optically transparent within a stated wavelength region and is known as an optical confinement layer. This transparent uneven layer is generally provided between the metal layer and a semiconductor active layer so that reflected light can effectively be utilized to improve short-circuit current density Jsc. Further, in order to prevent characteristics from lowering because of shunt pass, it is still also prevalent to provide between the metal layer and a semiconductor layer a layer formed of a light-transmitting material showing a conductivity, i.e., a transparent conductive layer. In general, these layers are deposited by a process such as vacuum evaporation or sputtering and show an improvement in short-circuit current density Jsc by 1 mA/cm2 or above.
As an example thereof, in xe2x80x9cOptical Confinement Effect in a-Si Solar Cells on 29p-MF-2 Stainless Steel Substratesxe2x80x9d (autumn, 1990), The 51st Applied Physics Society Scientific Lecture Meeting, Lecture Drafts p. 747, xe2x80x98P-IA-15a-SiC/a-Si/a-SiGe Multi-Bandgap Stacked Solar Cells with Bandgap Profilingxe2x80x99, Sannomiya et al., Technical Digest of The International PVSEC-5, Kyoto, Japan, p. 387, 1990, reflectance and texture structure were studied on a reflecting layer comprised of silver atoms. In this example, it is reported that the reflecting layer is deposited in a double layer of silver by changing substrate temperature, to form effective unevenness, which has achieved an increase in short-circuit current by virtue of an optical confinement effect.
The transparent layer used as an optical confinement layer is deposited by vacuum evaporation utilizing resistance heating or electron beams, sputtering, ion implantation or CVD (chemical vapor deposition). However, the facts of high wages for preparing target materials and so forth, a large repayment for vacuum apparatus and not a high utilization efficiency of materials make very high the cost for photovoltaic elements produced by these techniques, and put a high barrier to Industrial application of solar cells.
As a technique for forming a zinc oxide film by electrodeposition from an aqueous solution, intended to solve these problems. Japanese Patent Application Laid-Open No. 10-178193 discloses its combination with a metal layer and a transparent conductive layer which are formed by sputtering, applied as a reflecting layer of photovoltaic elements (solar cells). Also, as an improved technique of such a zinc oxide production technique, Japanese Patent Application Laid-Open No. 11-286799 by the present inventors discloses a zinc oxide film forming technique in which the roll-to-roll system is adopted to carry out successive electrodeposition on a long substrate.
These methods do not require any expensive vacuum apparatus and any expensive targets, and can dramatically reduce the production cost for zinc oxide films. These also enable deposition on a large-area substrate, and are full of promise for large-area photovoltaic elements such as solar cells.
However, these methods of making deposition electrochemically have the following problems.
That is, in an electrodeposition apparatus of the roll-to-roll system that holds a conductive long substrate above zinc, when the conductive long substrate is left to be dipped in an electrodeposition bath for an extended period of time from completion of one electrodeposition to subsequent electrodeposition, there are cases where deposition of zinc, zinc hydroxide or the like may occur to give rise to adsorption and to increase particles in the electrodeposition bath, thereby generating abnormal growth in the zinc oxide thin film. Further, metal components in the conductive long substrate may be dissolved into the electrodeposition bath.
Moreover, deposits or particles generated in the electrodeposition bath due to decrease of solubility by temperature lowering will accumulate on the zinc to degrade the uniformity of the film during the subsequent electrodeposition.
Further, when the conductive long substrate that adsorbs zinc, zinc hydroxide or the like is transported as such to a rinsing tank, the rinsing tank will be contaminated with particles, or rinsing failure or adhesion of particles to the surface of the zinc oxide thin film will occur.
In the production of a zinc oxide thin film by electrodeposition using the roll-to-roll system, any optimum electrodeposition apparatus capable of solving the above mentioned problems have not been provided.
Accordingly, the present invention has been accomplished taking account of the above mentioned problems, and an object of the present invention is to establish a novel technique for repeatedly using an electrodeposition bath in an electrodeposition apparatus for forming a zinc oxide thin film using the roll-to-roll system, to provide an apparatus and a process for producing a high-performance, low-cost zinc oxide thin film, and to contribute to real spread of the photovoltaic power generation by incorporating elements produced by the production apparatus and process into photovoltaic elements
According to a first aspect of the present invention, there is provided an apparatus for producing a zinc oxide film comprising an electrodeposition tank for holding an electrodeposition bath, at least one electrode comprised of zinc provided in the electrodeposition tank, transporting mechanism for transporting a conductive long substrate via above the electrode in the electrodeposition bath held in the electrodeposition tank, and a power source for applying an electric field between the electrode and the conductive long substrate, the apparatus further comprising means for bringing the conductive long substrate and the electrodeposition bath into non-contact state.
According to a second aspect of the present invention, there is provided an apparatus for producing a zinc oxide film comprising an electrodeposition tank for holding an electrodeposition bath, at least one electrode comprised of zinc provided in the electrodeposition tank, transporting mechanism for transporting a conductive long substrate via above the electrode in the electrodeposition bath held in the electrodeposition tank, and a power source for applying an electric field between the electrode and the conductive long substrate, the apparatus further comprising holding means for holding at least a part of the conductive long substrate above the electrodeposition bath.
According to a third aspect of the present invention, there is provided a process for producing a zinc oxide film comprising the steps of transporting a conductive long substrate via above at least one electrode comprised of zinc in an electrodeposition bath held in an electrodeposition tank and applying an electric field between the electrode and the conductive long substrate, thereby forming a zinc oxide film on the conductive long substrate, the process comprising:
a first step of forming the zinc oxide film on a part of the conductive long substrate;
a second step of stopping the application of the electric field and the transportation; and
a third step of bringing at least a part of a part of the conductive long substrate being in contact with the electrodeposition bath in the second step into non-contact with the electrodeposition bath.
As a preferred embodiment, the present invention provides the apparatus further comprising means for bringing the electrode and the electrodeposition bath into non-contact state.
As another preferred embodiment, the present invention provides the apparatus further comprising a circulation system connected to the electrodeposition tank, for circulating the electrodeposition bath, and a filter provided in the circulation system, for removing soil in the electrodeposition bath.
As still another preferred embodiment, the present invention provides the process further comprising after the third step, a fourth step of redipping in the electrodeposition bath the region as brought into non-contact with the electrodeposition bath in the third step, and a fifth step of restarting the application of the electric field and the transportation to form a zinc oxide film on the conductive long substrate.
As yet another preferred embodiment, the present invention provides the process wherein the water level of the electrodeposition bath is lowered in the third step.
As yet sill another preferred embodiment, the present invention provides the process wherein in the third step, at least a part of the part of the conductive long substrate being in contact with the electrodeposition bath is held by holding means provided above the electrodeposition bath to bring at least a region of the part of the conductive long substrate being in contact with the electrodeposition bath in the second step into non-contact with the electrodeposition bath.
As again another preferred embodiment, the present invention provides the process wherein the conductive long substrate comprises a conductive layer comprised of silver.
As still another preferred embodiment, the present invention provides the process wherein the electrodeposition bath contains zinc ions of 0.05 mol/L or more.